Dynamic shear and spall strengths of preliminarily quasi-statically extruded fine-grained uranium and U-0.3% Mo alloy

It is shown that substantial changes in the average grain size (by two orders of magnitude) and a twofold increase in the quasi-static yield stress and strength for uranium and 1.3-fold increase for the U-0.3% Mo alloy did not lead to a change in their shear strength upon a shock-wave loading. There is no correlation between the change in the shear and spall strength and the decrease in the average grain size. A tendency toward a regular increase (other conditions, i.e., amplitudes and the durations of the loading pulse, being equal) in the spall strength of materials in the row “extruded U, extruded U-0.3% Mo alloy, cast U-1.5% Mo alloy” was noted. The increase in the spall strength is connected with alloying and the real content of molybdenum in the alloy rather than with the effect of extrusion.     

Special features in convergence dynamics of steel shells under their explosive loading. Results of laser-interferometric measurements

The Fabry-Perot laser-interferometric diagnostics and the recovery method were used to study features of the convergence dynamics for high-radii shells of hardened 30KhGSA steel (HRC 35–40) and austenitic stainless 12Kh18N10T steel in the spherical explosive systems having the external radii R _HE = 40 and 110 mm. The experiments helped us to determine loading conditions that cause the following modes of convergence in steel shells: spall-free convergence, spall convergence with the closure of the spall with the main part of the shell, spall convergence without closure of spalls.     

Phase and structural transformations in U and U-Nb alloy upon severe deformation and heat treatments

Transmission electron microscopy was used to analyze the twin and dislocation structure of samples of commercial uranium in the initial (undeformed) state and after severe deformation using explosive loading by plane and spherical waves of various intensity. It has been shown that an increase in the intensity of explosive loading by a plane wave leads, first, to an increase in the density of randomly distributed dislocations and twins and, then, to the development of polygonization processes with the formation of a subgrain structure of the α phase. Crystallographic analysis of the initial and deformation-induced twins in uranium has shown the presence of predominantly {130} twins of mixed type and, in singular cases, {172} and {176} twins of the second kind. It has been established that the retained spherical shells have a distinctly pronounced zonal structure, which contains information on the forward and reverse martensitic phase transformations of uranium (α ? β(γ) ? L, etc.) that occur under shock-wave loading by spherical waves. Conditions are determined for the manifestation of structural heredity in the U-6 wt % Nb alloy with recovery of the size and shape of grains of the initial high-temperature γ phase during the forward γ → α″ martensitic transformation upon cooling and during reverse α″ → γ transformation upon heating. Elimination of the structural heredity with significant grain refinement of the high-temperature γ phase occurs in the process of repeated quenching from 700°C after one type of preliminary treatments (cold deformation of α″ martensite, recrystallization of the deformed α″ phase, high-temperature aging of the initial α″ martensite, and eutectoid decomposition).     

The influence of microstructure on the shock and spall behaviour of the magnesium alloy,Elektron 675

Alloying elements such as aluminium, zinc and rare earth metals allow precipitation hardening of magnesium (Mg). The low densities of such strengthened Mg alloys have led to their adoption as aerospace materials and (more recently) they are being considered as armour materials. Consequently, understanding their response to high strain-rate loading is becoming increasingly important. Here, the plate-impact technique was employed to measure stress evolution in an armour-grade wrought Mg alloy (Elektron 675) under one-dimensional shock loading. The effects of sample orientation and heat treatment were examined. The spall behaviour was interrogated using a heterodyne velocimeter system, with an estimate made of the material’s spall strength and Hugoniot elastic limit (HEL) for both aged and unaged materials. In particular, it is shown that the HEL and spall strength values are higher along the extrusion direction. It is thought that this is caused by striations of relatively small grains that run along the extrusion direction.     

Correlations Among Void Shape Distributions,Dynamic Damage Mode,and Loading Kinetics

Three-dimensional x-ray tomography (XRT) provides a nondestructive technique to characterize the size, shape, and location of damage in dynamically loaded metals. A shape-fitting method comprising the inertia tensors of individual damage sites was applied to study differences of spall damage development in face-centered-cubic (FCC) and hexagonal-closed-packed (HCP) multicrystals and for a suite of experiments on high-purity copper to examine the influence of loading kinetics on the spall damage process. Applying a volume-weighted average to the best-fit ellipsoidal aspect-ratios allows a quantitative assessment for determining the extent of damage coalescence present in a shocked metal. It was found that incipient transgranular HCP spall damage nucleates in a lenticular shape and is heavily oriented along particular crystallographic slip directions. In polycrystalline materials, shape distributions indicate that a decrease in the tensile loading rate leads to a transition to coalesced damage dominance and that the plastic processes driving void growth are time dependent.     

Mechanical properties of grade M1 copper before and after shock compression in a wide range of loading durations

The effect of the initial microstructure and microstructure obtained after quasi-isentropic and shock compression on the elastic-plastic and strength characteristics of grade M1 copper upon static, quasistatic, and dynamic loading has been investigated. It has been revealed that the parameters of a shock wave play an important role in the formation of the substructure and related mechanical properties of the samples and that the values of the elastic-plastic and strength characteristics in coarse-crystalline samples of copper M1 vary substantially depending on the defect structure at the identical grain size. Measurements of the elastic limit and critical fracture stresses over a wide range of the loading durations have been performed by different methods, including a VISAR laser interferometer. Based on the experimental data obtained, models of the shear and spall strength of copper for different loading conditions have been developed. With the aid of a NAG two-stage kinetic model, a numerical simulation of the dynamic fracture of coarse-crystalline samples of copper M1 with different internal structure has been performed. An analysis of the experiments in combination with the numerical simulation made it possible to describe the deformation behavior of the samples in the entire range of loading rates.     

COSP: A computer model of cyclic oxidation

A computer model useful in predicting the cyclic oxidation behavior of alloys is presented. The model considers the oxygen uptake due to scale formation during the heating cycle and the loss of oxide due to spalling during the cooling cycle. The balance between scale formation and scale loss is modeled and used to predict weight change and metal loss kinetics. A simple uniform spalling model is compared to a more complex random spall site model. In nearly all cases, the simpler uniform spall model gave predictions as accurate as the more complex model. The model has been applied to several nickel-base alloys which, depending upon composition, form Al₂O₃ or Cr₂O₃ during oxidation. The model has been validated by several experimental approaches. Versions of the model that run on a personal computer are available.     

Effects of Two Different Dynamic Loading Conditions on Spall and Damage of 7075 Aluminum Alloy

The fracture behaviors of the 7075 aluminum alloy under two different dynamic loading conditions are investigated by means of a light-gas gun. The fracture surfaces obtained in the spall test are compared to the fracture surfaces obtained with a blunt projectile struck to the aluminum alloy plate. Optical and scanning electron microscopes are used in the investigation. For the plate-impact test, spall of the target was attributed to intergranular fracture caused by the tensile stress. The fracture behavior during projectile penetration is complex and consists of several fracture modes in addition to that the fracture is also of dynamic character. The penetration process of aluminum alloy target included: plugging stage, the microcracks nucleation stage, and the final tensile fracture stage. Mixed intergranular brittle/ductile fracture was observed, and brittle fracture played a dominate role.     

爆炸焊接装药厚度可焊性窗口

通过X射线观测爆炸焊接复板的运动姿态,得出了爆轰载荷与复板的作用过程,从而提出了在爆炸焊接过程中"爆轰载荷产生的复板的最大弯矩必须大于复板材料在其动态屈服极限时的弯矩而小于其在动态抗拉极限时的弯矩,才能实现成功爆炸焊接"这一新观点,并由此得出了爆炸焊接装药厚度的上限和下限,此即为爆炸焊接装药厚度可焊性窗口.在此基础上,...     

Micromechanics of spall and damage in tantalum

We conducted a number of plate impact experiments using an 80-mm launcher to study dynamic void initiation, linkup, and spall in tantalum. The tests ranged in shock pressure so that the transition from void initiation, incipient spall, and full spall could be studied. Wave profiles were measured using a velocity interferometry system (VISAR), and targets were recovered using “soft” recovery techniques. We utilized scanning electron microscopy, metallographic cross-sections, and plateau etching to obtain quantitative information concerning damage evolution in tantalum under spall conditions. The data (wave profiles and micrographs) are analyzed in terms of a new theory and model of dynamic damage cluster growth.     

圆筒内嵌钨合金球爆炸加载条件下断裂机制研究

针对钨合金作为预制破片战斗部穿甲后的易碎性,采用圆筒内嵌钨合金球战斗部,研究了97.5W-Ni-Fe合金在圆筒内嵌式爆炸加载穿靶前后的微观组织以及断裂机制。结果表明,97.5W-Ni-Fe合金爆炸加载后在钨颗粒内部产生大量形变孪晶,穿透靶板后形变孪晶诱发大量微观裂纹,微裂纹在穿靶后的拉应力作用下扩展并与W-W界面断裂相互连接,使钨合金球断裂成有效破片,增强了钨合金的二次毁伤能力。     

Failure mode and fatigue mechanism of laser-remelted plasma-sprayed Ni alloy coatings in rolling contact

The failure mode and rolling contact fatigue failure mechanism of the laser-melted plasma-sprayed coatings were investigated in this paper. The coating was deposited by using a high-efficiency plasma spraying system and remelted by using a CO₂ laser in a continuous mode. Thirteen rolling contact tests were performed to obtain the statistical result at an identical condition. Experimental results showed that spalling is the main failure mode of the remelted coating in rolling contact. Prior to the formation of the spall, the surface main crack and ring-type crack have been generated. However, only the main crack might not directly cause the formation of the spall. The joining of the ring-type cracks and subsurface branched cracks was directly responsible for the spall formation.     

Formation and structure of vortex zones arising upon explosion welding of carbon steels

Presented are the results of investigation of vortex zones arising upon explosion welding of thin plates of steel 20. Specific features of the structure of the vortices and zones of the deformed material adjacent to them have been revealed by methods of structure analysis. It has been shown that in the process of explosive loading the central regions of the vortices characterized by an enhanced carbon content were in the molten state. The microhardness in the region of vortex zones reaches 5700 MPa. The character of the arrangement of ferrite grains and martensite microvolumes in peripheral regions of vortices is caused by intense rotation of the material. The intense intermixing of materials in different states of aggregation in vortex zones is one of the factors responsible for the formation of cavities, whose volume exceeds the volume shrinkage occurring upon casting of carbon steels. It has been established that traces of vortex zones are retained even after one-hour annealing of welded packets at 800°C.     

Laser shock-induced spalling and fragmentation in vanadium

Polycrystalline and monocrystalline (〈1 0 0〉 and 〈1 1 0〉) vanadium was subjected to shock compression followed by tensile wave release to study spall and fragmentation behavior. The shock pulse was generated by a direct laser drive at energy levels ranging from 11 to 440 J mm^–2 (laser beam irradiated area 1.12 mm²) and initial pulse durations of 3 and 8 ns (approximate initial pressures between 10 and 250 GPa). Glass and polycarbonate shields placed at a specific distance behind the vanadium targets were used to collect and analyze the ejected fragments in order to evaluate and quantify the extent of damage. The effects of target thickness, laser energy, polycrystallinity and pulse duration were studied. Calculations show melting at a pressure threshold of ～150 GPa, which corresponds to a laser energy level of ～180 J mm^–2. Consistent with the analytical predictions, the recovered specimens and fragments show evidence of melting at the higher energy levels. Spalling in the polycrystals occurred by a ductile tearing mechanism that favored grain boundaries. In the monocrystals it occurred by a mixture of cleavage fracture along the {0 1 0} planes and ductile dimple fracture. This lower spall strength in polycrystals contradicts predictions from the Hall–Petch equation. Experimentally obtained fragment sizes were compared with predictions from the Grady–Kipp model. The spall strength of vanadium under laser loading conditions was calculated from both VISAR pull-back signals and using the spall thickness. It was found to be considerably higher than predictions from gas gun experiments, the monocrystals showing a higher value than polycrystals. This higher spall strength is suggestive of a strong time dependence of the phenomenon, consistent with the nucleation and growth kinetics of voids and the strain rate sensitivity embedded in the Grady theory.     

Effects of the Presence of Water Vapour on the Oxidation Behaviour of Low Carbon–Low Silicon Steel in 1 %O2–N2 at 1073 K

The effect of water vapour additions in the range of 2.5–24.8 % on the oxidation behaviour of a low carbon and low silicon steel in 1 %O₂–N₂ at 1073 K (800 °C) was examined. It was found that the characteristic of steel oxidation was completely changed by addition of water vapour in the atmosphere. First, the kinetics was changed from non-parabolic to parabolic. Second, the scale formed in 1 %O₂–N₂ for 30 min or longer was easy to spall upon cooling whereas the scales formed in the water-vapour containing atmospheres did not spall easily. Third, additions of 2.5–10 % of water vapour in 1 %O₂–N₂ resulted in the formation of numerous depressed locations in the scale, but the scale-steel adherence in the areas surrounding the depressed locations was very much strengthened. Finally, additions of 17.2 and 24.8 % of water vapour in the atmosphere prevented both scale spallation upon cooling and formation of depressed areas in the scale. The mechanisms of forming various scale structures and the roles of water vapour additions under different conditions were discussed.     

高应变率下铀铌合金的断裂组织特征

对经过内部爆破加载后产生的U-Nb合金破片进行了断口形貌、金相组织等微观分析。结果表明：U-Nb合金在爆炸加载下的断裂方式为剪切断裂；破片组织内部发现因高速应变引起的大量的绝热剪切带，靠近绝热剪切带的基体晶粒发生明显的拉长变形：破片中存在大量的微裂纹，裂纹沿着绝热剪切带扩展，当与沿壳体环向的拉应力形成的纵向主裂纹相交汇时，形成破片。     

Types of twins and the dislocation structure of α uranium in the initial and explosion-deformed states

Transmission electron microscopy is used to study and analyze the twin and dislocation structures of commercial-purity uranium samples in the initial (undeformed) state and after severe deformation induced by shock loading by plane waves with various intensities. As the shock loading intensity increases, the density of chaotically distributed dislocations and twins first increases, and, then, polygonization processes develop and result in a subgrain structure. Crystallographic analysis of the initial and deformation twins in uranium reveals predominant twins of the compound type {130} and rare {172} and {176} second-type twins.     

钛/铝爆炸焊接界面扩散行为分子动力学模拟

为揭示钛/铝爆炸焊接界面原子的扩散行为,采用分子动力学模拟从原子尺度分析了钛/铝爆炸焊接界面原子的微观扩散机理。利用Materials Studio建立了钛/铝爆炸焊接焊点处的分子动力学模型,结合爆炸焊接的物理过程,将爆炸焊接过程分为加载和卸载2个阶段,通过LAMMPS程序计算了爆炸焊接钛、铝原子的均方位移、径向分布函数、扩散层厚度等,利用OVITO软件再现了不同阶段界面原子的扩散行为。在爆炸焊接加载阶段,钛、铝原子不发生扩散,只在平衡位置做振动,铝原子振动要比钛原子振动强。爆炸焊接卸载开始时,钛、铝原子发生互扩散。钛/钛原子键能高,不易破坏,铝/铝原子键能低,容易破坏产生空位、间隙等缺陷,有利于钛原子深入扩散到铝晶格内部,但铝原子难以进入钛的晶格内部。采用扫描电镜和EDS能谱表征了钛/铝爆炸焊接复合材料界面元素分布,与模拟结果有很好的一致性。     

EXPERIMENTAL INVESTIGATION ON GENERATION MECHANISM OF EXPLOSIVE STRESS WAVES

１ＩＮＴＲＯＤＵＣＴＩＯＮＥｘｐｌｏｓｉｖｅｓｔｒｅｓｓｗａｖｅｓａｒｅｑｕｉｔｅｉｍｐｏｒｔａｎｔｉｎｒｏｃｋｆｒａｇｍｅｎｔａｔｉｏｎｐｒｏｃｅｓ．Ｍａｎｙｓｃｈｏｌａｒｓｈａｖｅｉｎｖｅｓｔｉｇａｔｅｄｔｈｅｇｅｎｅｒａｔｉｏｎａｎｄｐｒｏｐａ...     

NEW ACHIEVEMENTS ON THE THEORY AND TECHNOLOGY OF EXPLOSIVE WELDING

There are four new achievements of this work on the theory and technology of explosive welding.(1) It has been found and defined three kinds of bonding interfaces: big wavy, small wavy and micro wavy, and the micro wavy interface is the best. In a cladding plate, it is for the first time to find that the form of interface presents regular distribution.(2) Although the interface has the features of melt, diffusion and pressure welding in the mean time, the seam and "hole" brought by the melt weaken the bonding strength of interface greatly, and the effect of melt on interface must be eliminated in explosive welding, so explosive welding is not a melt weld. The diffusion welding is a kind of form of pressure welding, and the diffusion is not the reason of the bonding of interface but the result of interface high pressure. So the diffusion welding cannot also explain the bonding mechanism of it. The experiment and theory make clear that explosive welding is a special pressure one.(3) To get good interface